Extensive research has been carried out on improving the properties required for tire covers. In connection with the permanent objective of reducing the fuel consumption of motor vehicles, efforts are thus made to improve the rolling resistance of said covers, while still maintaining excellent grip both to dry surfaces and to wet surfaces, as well as suitable wear resistance.
Elastomeric compositions intended to form these treads are already known. They generally comprise, apart from a diene elastomer, a reinforcing filler. This reinforcing filler is generally carbon black or silica.
To obtain optimum reinforcement properties conferred by the filler, it is known that it is necessary for the filler to be present in the elastomer matrix in a final form that is both as finely divided as possible and as uniformly distributed as possible. However, such conditions cannot be met except if the filler has a very high capability, on the one hand, of being incorporated into the matrix during mixing with the elastomer and, on the other hand, of being uniformly dispersed within said matrix.
As is known, carbon black has such a capability, but this is not the case in general for silica particles. This is because, for reciprocal affinity reasons, silica particles have an annoying tendency to form agglomerations in the elastomer matrix. This has a deleterious effect on the reinforcing properties of the final product obtained after vulcanization. It is also problematic insofar as, by increasing the viscosity of the elastomer, silica, after it has been incorporated, makes the vulcanizable elastomeric composition difficult to work or to compound, especially when the non-sulfur-containing additives and the vulcanization system are being incorporated by mixing, or when the compound containing all the ingredients is being introduced into a vulcanization mold.
However, the use of silica in the manufacture of tire treads is particularly advantageous insofar as it is able to lower the rolling resistance and therefore give a substantial fuel saving.
The prior art has already described products that promote the dispersion of a filler, especially silica, in an elastomeric composition. Such products are often called coupling agents.
Thus, European patent application EP 0 501 227 mentions, for a sulfur-vulcanizable composition comprising a conjugated-diene copolymer reinforced by silica, the use of 12.8 parts by weight (per 100 parts of rubber) of a reinforcing agent comprising 6.4 parts by weight of a polysulfidic organosilane.
International application WO 97/42256 also mentions, for elastomeric compositions containing silica, the use as coupling agent of a specific polysulfidic organosilane, namely bis(triethyoxysilylpropyl)tetrasulfide, also known by the brand name Si-69® from Degussa AG. This coupling agent, which is widely used in industry, does however have the drawback of being extremely expensive.
This is the reason why the above document teaches, essentially, elastomeric compositions containing treated carbon black as reinforcing filler, for which it recommends the use, as prevulcanization modifier, of a non-organosilane polysulfide compound, for example the use of a tert-butylphenol polysulfide.
International application WO 02/083719 again deals with the problem of replacing bis(triethoxysilylpropyl)tetrasulfide as coupling agent with a white filler (such as silica) in a composition based on diene rubbers. For this purpose, it proposes polysulfidic monoorganoxysilanes. However, the latter compounds also cause excessive costs because of the chemistry used.